I had this problem too. What you're doing is once you see your landing at 360, you stop turning your head. You gotta ignore the landing completely on your first spin and catch it at 630 degrees. If you have a gopro or helmet cam, you can see it. It also doesn't hurt to set your spin too hard because you can always open up to slow it down. when I first learned them, it was on a 25' jump so it was huck city. Now that I do them on bigger jumps, it makes the spin easier.
Hope this helps.